This invention relates to the art of electric arc welding and, more particularly, to a nozzle for a welding gun for shielded arc welding in a confined environment in which the gas nozzle assembly of the nozzle is subjected to extremely high temperatures.
In the field of gas shielded metal arc welding, it is of course well known to provide a welding gun having a nozzle through which a welding wire electrode and gas are fed coaxially towards workpieces to be welded. Such a welding gun nozzle generally includes a gas nozzle assembly through which the electrode and gas freely pass toward a workpiece with the electrode being electrically energized at the outlet end of the nozzle assembly. The shielding gas is diffused into a cylindrical stream around the electrode as it emerges from the outlet end of the assembly.
A nozzle for a shielded arc welding gun in accordance with the present invention finds particular utility in conjunction with the welding together of closely spaced apart longitudinally adjacent railway rails and under conditions which subject the gas nozzle assembly thereof to extremely high and potentially damaging temperatures. Accordingly, the invention is described herein in detail in connection with such use. At the same time, however, it will be appreciated that a nozzle according to the invention can be used other than for welding railroad rails and, in particular, can be used in welding workpieces in other confining environments in which the gas nozzle assembly of the nozzle is subjected to high temperatures.
In co-pending application Ser. No. 950,408 filed Oct. 15, 1997 and assigned to the same assignee as the present application, now U.S. Pat. No. 5,773,779, issued Jun. 30, 1998, and the subject matter of which co-pending application is hereby incorporated herein by reference for purposes of background information, shielded arc welding processes are disclosed for installing and repairing railroad rails in the field by joining the ends of longitudinally adjacent rails. The rails to be joined are longitudinally spaced apart about one inch and are of standard cross-sectional shape and accordingly have a laterally extending lower base which is relatively wide and a web extending upwardly from the base and merging into an upper wheel engaging head. Preferably, a steel barrier plate is wedged between the lower ends of the bases of the adjacent rails, the rails are preheated to a temperature of about 900-1,000.degree. F. and the gap between the rails is filled with molten metal by the gas shielded arc welding process which is initiated by bringing a welding gun nozzle downwardly into the gap between the rails until the electrode of the gun contacts the barrier plate to initiate the welding process. The nozzle of the welding gun is moved laterally back and forth across the gap between the rails to fill the gap between the bases of the rails and, when the welding process approaches the gap between the web portions of the rails, contoured copper shoes are used to close the gap between the webs and heads so that the latter gap provides an enclosed cavity. This cavity is filled by continuing the shielded gas welding process referred to hereinabove and by which beads of filler metal are continuously laid sequentially in laterally opposite directions between the laterally opposite sides of the webs and heads of the rails.
A major problem encountered in connection with the welding of railroad rails through the use of shielded arc welding gun equipment heretofore available is the difficulty of completing the welding process without the gas nozzle assembly of the gun being damaged or destroyed by the extreme heat generated by the welding process. In particular, the heat in the gap between the webs and heads basically is dissipated only from the opening at the top edges of the heads due to the enclosed cavity created by the copper shoes extending vertically along the laterally opposite ends of the webs and heads of the rails. Further, the gas nozzle assembly is initially exposed or subjected to a high temperature as the result of the preheating of the rails, and the copper shoes provide a chimney effect which not only confines the heat in the cavity defined by the rail ends and shoes but also restricts dissipation of the heat through the upper end of the cavity, whereby the gas nozzle assembly is subjected to the heat flowing upwardly thereacross to the upper end of the cavity. The cavity additionally has a vertical depth of more than six inches whereby it will be appreciated that the nozzle assembly is subjected to considerable heat, which can reach over 2,000.degree. F., over a considerable period of time as the nozzle assembly is laterally translated back and forth in the gap to progressively lay the beads of molten metal necessary for filling the gap and joining the rails. With shielded arc welding guns heretofore available, melting of the gas nozzle assembly or parts thereof, and thus destruction of the latter can occur prior to completion of the welding process. Partially in this respect, and in addition to the confined space from which heat dissipation is limited, the fact that the gap between the longitudinally adjacent rails is about one inch necessitates the nozzle assembly having a small outer diameter, generally about 5/8 inch. Accordingly, the outer jacket or gas shroud component of the nozzle assembly is extremely thin walled as are the other component parts of the nozzle assembly. Moreover, the electrical insulation provided between the outer copper shroud and the inner copper gas diffuser retards the transfer of heat from the shroud and this, together with inadequate cooling of the gas nozzle assembly, results in the parts of the gas nozzle assembly and especially the gas shroud being quickly heated and having little resistance to the damaging effect of high temperatures, including melting thereof. Melting of the gas nozzle assembly, or parts thereof, is not only costly from the standpoint of the loss of equipment but also from the standpoint of lost time with respect to completing the process and the likelihood of a loss in the quality of the welded joint resulting from the interruption of the process. In this respect, in order to provide a sound metal interface between the ends of the rails and the weld metal produced by the electrode, the laying of the successive beads must be continuous throughout the process.